Elastic wave device and method for manufacturing the same

ABSTRACT

An elastic wave device includes a supporting substrate, a high-acoustic-velocity film stacked on the supporting substrate and in which an acoustic velocity of a bulk wave propagating therein is higher than an acoustic velocity of an elastic wave propagating in a piezoelectric film, a low-acoustic-velocity film stacked on the high-acoustic-velocity film and in which an acoustic velocity of a bulk wave propagating therein is lower than an acoustic velocity of a bulk wave propagating in the piezoelectric film, the piezoelectric film is stacked on the low-acoustic-velocity film, and an IDT electrode stacked on a surface of the piezoelectric film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an elastic wave device preferably foruse in a resonator, a bandpass filter, or the like and a method formanufacturing the same. More particularly, the present invention relatesto an elastic wave device having a structure including a supportingsubstrate, a piezoelectric layer, and a layer of another materialdisposed therebetween, and a method for manufacturing the same.

2. Description of the Related Art

Elastic wave devices have been widely used as resonators and bandpassfilters, and in recent years, there has been a need for increasing thefrequency thereof. Japanese Unexamined Patent Application PublicationNo. 2004-282232 described below discloses a surface acoustic wave devicein which a hard dielectric layer, a piezoelectric film, and an IDTelectrode are stacked in that order on a dielectric substrate. In such asurface acoustic wave device, by disposing the hard dielectric layerbetween the dielectric substrate and the piezoelectric film, an increasein the acoustic velocity of surface acoustic waves is achieved. It isdescribed that thereby, the frequency of the surface acoustic wavedevice can be increased.

Japanese Unexamined Patent Application Publication No. 2004-282232 alsodiscloses a structure in which a potential equalizing layer is providedbetween the hard dielectric layer and the piezoelectric film. Thepotential equalizing layer is composed of a metal or semiconductor. Thepotential equalizing layer is provided in order to equalize thepotential at the interface between the piezoelectric film and the harddielectric layer.

In the surface acoustic wave device described in Japanese UnexaminedPatent Application Publication No. 2004-282232, an increase in acousticvelocity is achieved by forming the hard dielectric layer. However,there is considerable propagation loss, and surface acoustic wavescannot be effectively confined within the piezoelectric thin film.Consequently, the energy of the surface acoustic wave device leaks intothe dielectric substrate, and therefore, it is not possible to enhancethe Q factor, which is a problem.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide an elastic wavedevice having a high Q factor and a method for manufacturing the same.

An elastic wave device including a piezoelectric film according to apreferred embodiment of the present invention includes ahigh-acoustic-velocity supporting substrate in which the acousticvelocity of a bulk wave propagating therein is higher than the acousticvelocity of an elastic wave propagating in the piezoelectric film; alow-acoustic-velocity film stacked on the high-acoustic-velocitysupporting substrate, in which the acoustic velocity of a bulk wavepropagating therein is lower than the acoustic velocity of a bulk wavepropagating in the piezoelectric film; the piezoelectric film stacked onthe low-acoustic-velocity film; and an IDT electrode disposed on asurface of the piezoelectric film.

The elastic wave device including a piezoelectric film according to apreferred embodiment of the present invention is structured such thatsome portion of energy of an elastic wave propagating in thepiezoelectric film is distributed into the low-acoustic-velocity filmand the high-acoustic-velocity supporting substrate.

An elastic wave device including a piezoelectric film according to apreferred embodiment of the present invention includes a supportingsubstrate; a high-acoustic-velocity film disposed on the supportingsubstrate, in which the acoustic velocity of a bulk wave propagatingtherein is higher than the acoustic velocity of an elastic wavepropagating in the piezoelectric film; a low-acoustic-velocity filmstacked on the high-acoustic-velocity film, in which the acousticvelocity of a bulk wave propagating therein is lower than the acousticvelocity of a bulk wave propagating in the piezoelectric film; thepiezoelectric film stacked on the low-acoustic-velocity film; and an IDTelectrode disposed on a surface of the piezoelectric film.

The elastic wave device including a piezoelectric film according to apreferred embodiment of the present invention is structured such thatsome portion of energy of an elastic wave propagating in thepiezoelectric film is distributed into the low-acoustic-velocity filmand the high-acoustic-velocity film.

In a specific aspect of the elastic wave device according to a preferredembodiment of the present invention, the low-acoustic-velocity film ispreferably made of silicon oxide or a film containing as a majorcomponent silicon oxide. In such a case, the absolute value of thetemperature coefficient of frequency TCF can be decreased. Furthermore,the electromechanical coupling coefficient can be increased, and theband width ratio can be enhanced. That is, an improvement in temperaturecharacteristics and an enhancement in the band width ratio can besimultaneously achieved.

In another specific aspect of the elastic wave device according to apreferred embodiment of the present invention, the thickness of thepiezoelectric film preferably is about 1.5λ or less, for example, whereλ is the wavelength of an elastic wave determined by the electrodeperiod of the IDT electrode. In such a case, by selecting the thicknessof the piezoelectric film in a range of about 1.5λ or less, for example,the electromechanical coupling coefficient can be easily adjusted.

In another specific aspect of the elastic wave device according to apreferred embodiment of the present invention, by selecting thethickness of the piezoelectric film in a range of about 0.05, to about0.5λ, where λ is the wavelength of an elastic wave determined by theelectrode period of the IDT electrode, the electromechanical couplingcoefficient can be easily adjusted over a wide range.

In another specific aspect of the elastic wave device according to apreferred embodiment of the present invention, the thickness of thelow-acoustic-velocity film preferably is a preferred embodiment of 2λ orless, for example, where λ is the wavelength of an elastic wavedetermined by the electrode period of the IDT electrode. In such a case,by selecting the thickness of the low-acoustic-velocity film in a rangeof about 2λ or less, for example, the electromechanical couplingcoefficient can be easily adjusted. Furthermore, the warpage of theelastic wave device due to the film stress of the low-acoustic-velocityfilm can be reduced. Consequently, freedom of design can be increased,and it is possible to provide a high-quality elastic wave device whichis easy to handle.

In another specific aspect of the elastic wave device according to apreferred embodiment of the present invention, the piezoelectric film ispreferably made of single-crystal lithium tantalate with Euler angles(0±5°, θ, ψ), and the Euler angles (0±5°, θ, ψ) are located in any oneof a plurality of regions R1 shown in FIG. 17. In such a case, theelectromechanical coupling coefficient of the SH component of theelastic wave can be set at about 2% or more, for example. Consequently,the electromechanical coupling coefficient can be sufficientlyincreased.

In another specific aspect of the elastic wave device according to apreferred embodiment of the present invention, the piezoelectric film ispreferably made of single-crystal lithium tantalate with Euler angles(0±5°, θ, ψ), and the Euler angles (0±5°, θ, ψ) are located in any oneof a plurality of regions R2 shown in FIG. 18. In such a case, theelectromechanical coupling coefficient of the SH component used can beincreased, and the SV wave, which is spurious, can be effectivelysuppressed.

In another specific aspect of the elastic wave device according to apreferred embodiment of the present invention, the coefficient of linearexpansion of the supporting substrate is lower than that of thepiezoelectric film. In such a case, the temperature characteristics canbe further improved.

In another specific aspect of the elastic wave device according to apreferred embodiment of the present invention, the specific acousticimpedance of the low-acoustic-velocity film is lower than that of thepiezoelectric film. In such a case, the band width ratio can be furtherenhanced.

In another specific aspect of the elastic wave device according to apreferred embodiment of the present invention, a dielectric film isdisposed on the piezoelectric film and the IDT electrode, and a surfaceacoustic wave propagates in the piezoelectric film.

In another specific aspect of the elastic wave device according to apreferred embodiment of the present invention, a dielectric film isdisposed on the piezoelectric film and the IDT electrode, and a boundaryacoustic wave propagates along a boundary between the piezoelectric filmand the dielectric film.

In another specific aspect of the elastic wave device according to apreferred embodiment of the present invention, at least one of anadhesion layer, an underlying film, a low-acoustic-velocity layer, and ahigh-acoustic-velocity layer is disposed in at least one of boundariesbetween the piezoelectric film, the low-acoustic-velocity film, thehigh-acoustic-velocity film, and the supporting substrate.

A method for manufacturing an elastic wave device according to yetanother preferred embodiment of the present invention includes a step ofpreparing a supporting substrate; a step of forming ahigh-acoustic-velocity film, in which the acoustic velocity of a bulkwave propagating therein is higher than the acoustic velocity of anelastic wave propagating in a piezoelectric, on the supportingsubstrate; a step of forming a low-acoustic-velocity film, in which theacoustic velocity of a bulk wave propagating therein is lower than theacoustic velocity of a bulk wave propagating in a piezoelectric, on thehigh-acoustic-velocity film; a step of forming a piezoelectric layer onthe low-acoustic-velocity film; and a step of forming an IDT electrodeon a surface of the piezoelectric layer.

In a specific aspect of the method for manufacturing an elastic wavedevice according to a preferred embodiment of the present invention, thesteps of forming the high-acoustic-velocity film, thelow-acoustic-velocity film, and the piezoelectric layer on thesupporting substrate include the following steps (a) to (e).

(a) A step of performing ion implantation from a surface of apiezoelectric substrate having a larger thickness than that of thepiezoelectric layer.

(b) A step of forming the low-acoustic-velocity film on the surface ofthe piezoelectric substrate on which the ion implantation has beenperformed.

(c) A step of forming the high-acoustic-velocity film on a surface ofthe low-acoustic-velocity film, opposite to the piezoelectric substrateside of the low-acoustic-velocity film.

(d) A step of bonding the supporting substrate to a surface of thehigh-acoustic-velocity film, opposite to the surface on which thelow-acoustic-velocity film is stacked.

(e) A step of, while heating the piezoelectric substrate, separating apiezoelectric film, at a high concentration ion-implanted region of thepiezoelectric substrate in which the implanted ion concentration ishighest, from a remaining portion of the piezoelectric substrate suchthat the piezoelectric film remains on the low-acoustic-velocity filmside.

In another specific aspect of the method for manufacturing an elasticwave device according to a preferred embodiment of the presentinvention, the method further includes a step of, after separating theremaining portion of the piezoelectric substrate, heating thepiezoelectric film disposed on the low-acoustic-velocity film to recoverpiezoelectricity. In such a case, since the piezoelectricity of thepiezoelectric film can be recovered by heating, it is possible toprovide an elastic wave device having good characteristics.

In another specific aspect of the method for manufacturing an elasticwave device according to a preferred embodiment of the presentinvention, the method further includes a step of, prior to bonding thesupporting substrate, performing mirror finishing on the surface of thehigh-acoustic-velocity film, opposite to the low-acoustic-velocity filmside of the high-acoustic-velocity film. In such a case, it is possibleto strengthen bonding between the high-acoustic-velocity film and thesupporting substrate.

In the elastic wave device according to various preferred embodiments ofthe present invention, since the high-acoustic-velocity film and thelow-acoustic-velocity film are disposed between the supporting substrateand the piezoelectric film, the Q factor can be enhanced. Consequently,it is possible to provide an elastic wave device having a high Q factor.

Furthermore, in the manufacturing method according to various preferredembodiments of the present invention, it is possible to provide anelastic wave device of the present invention having a high Q factor.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic elevational cross-sectional view of a surfaceacoustic wave device according to a first preferred embodiment of thepresent invention, and FIG. 1B is a schematic plan view showing anelectrode structure of the surface acoustic wave device.

FIG. 2 is a graph showing impedance characteristics of surface acousticwave devices of the first preferred embodiment, a first comparativeexample, and a second comparative example.

FIG. 3 shows an impedance Smith chart for the surface acoustic wavedevices of the first preferred embodiment, the first comparativeexample, and the second comparative example.

FIGS. 4A and 4B are each a graph showing the results of simulation inthe IDT electrode portion of the surface acoustic wave device of thefirst preferred embodiment of the present invention, regarding therelationship between the AlN film thickness and the percentage of energyconcentration.

FIG. 5 is a graph showing the results of FEM simulation of impedancecharacteristics of surface acoustic wave devices according to the firstpreferred embodiment and the first and second comparative examples.

FIG. 6 is a graph showing the results of FEM simulation of therelationship between the Q factor and the frequency in the surfaceacoustic wave devices of the first preferred embodiment and the firstand second comparative examples.

FIG. 7 is a graph showing the relationship between the thickness of thepiezoelectric film composed of LiTaO₃, the acoustic velocity, and thenormalized film thickness of the low-acoustic-velocity film composed ofSiO₂ in a surface acoustic wave device according to a preferredembodiment of the present invention.

FIG. 8 is a graph showing the relationship between the thickness of thepiezoelectric film composed of LiTaO₃, the normalized film thickness ofthe low-acoustic-velocity film composed of SiO₂, and theelectromechanical coupling coefficient in the surface acoustic wavedevice according to a preferred embodiment of the present invention.

FIG. 9 is a graph showing the relationship between the thickness of thepiezoelectric film composed of LiTaO₃, the normalized film thickness ofthe low-acoustic-velocity film composed of SiO₂, and the TCV in asurface acoustic wave device according to a preferred embodiment of thepresent invention.

FIG. 10 is a graph showing the relationship between the thickness of thepiezoelectric film composed of LiTaO₃, the normalized film thickness ofthe low-acoustic-velocity film composed of SiO₂, and the band widthratio in a surface acoustic wave device according to a preferredembodiment of the present invention.

FIG. 11 is a graph showing the relationship between the band width ratioBW and the TCF in surface acoustic wave devices of third to fifthcomparative examples.

FIG. 12 is a graph showing the relationship between the band widthratio, the temperature coefficient of frequency TCV, and the normalizedfilm thickness of the low-acoustic-velocity film in the surface acousticwave device of the first preferred embodiment of the present invention.

FIG. 13 is a graph showing the relationship between the thickness of thepiezoelectric film composed of LiTaO₃ and the acoustic velocity at theresonance point as well as the acoustic velocity at the antiresonancepoint in the surface acoustic wave device according to a secondpreferred embodiment of the present invention.

FIG. 14 is a graph showing the relationship between the thickness of thepiezoelectric film composed of LiTaO₃ and the band width ratio in thesurface acoustic wave device according to the second preferredembodiment of the present invention.

FIG. 15 is a graph showing the relationship between the normalized filmthickness of the SiO₂ film and the material for thehigh-acoustic-velocity film in surface acoustic wave devices accordingto a third preferred embodiment of the present invention.

FIG. 16 is a graph showing the relationship between the normalized filmthickness of the SiO₂ film, the electromechanical coupling coefficient,and the material for the high-acoustic-velocity film in surface acousticwave devices according to the third preferred embodiment of the presentinvention.

FIG. 17 is a graph showing a plurality of regions R1 in which theelectromechanical coupling coefficient of the surface acoustic wave modecontaining as a major component the U2 (SH) mode preferably is about 2%or more in the LiTaO₃ films with Euler angles (0±5°, θ, ψ) of surfaceacoustic wave devices according to a fourth preferred embodiment of thepresent invention.

FIG. 18 is a graph showing a plurality of regions R2 in which theelectromechanical coupling coefficient of the surface acoustic wave modecontaining as a major component the U2 mode preferably is about 2% ormore and the electromechanical coupling coefficient of the surfaceacoustic wave mode containing as a major component the U3 (SV) mode,which is spurious, preferably is about 1% or less in the LiTaO₃ filmswith Euler angles (0±5°, θ, ψ) of surface acoustic wave devicesaccording to the fifth preferred embodiment of the present invention.

FIGS. 19A to 19C are graphs showing the relationships between thespecific acoustic impedance of the low-acoustic-velocity film and theband width ratio in surface acoustic wave devices according to a sixthpreferred embodiment of the present invention.

FIGS. 20A to 20C are graphs showing the relationships between thespecific acoustic impedance of the low-acoustic-velocity film and theacoustic velocity of the surface acoustic wave in surface acoustic wavedevices according to the sixth preferred embodiment of the presentinvention.

FIGS. 21A to 21E are elevational cross-sectional views for explaining amethod for manufacturing a surface acoustic wave device according to thefirst preferred embodiment of the present invention.

FIGS. 22A to 22C are elevational cross-sectional views for explainingthe method for manufacturing a surface acoustic wave device according tothe first preferred embodiment of the present invention.

FIG. 23 is a schematic elevational cross-sectional view of a surfaceacoustic wave device according to a seventh preferred embodiment of thepresent invention.

FIG. 24 is a schematic elevational cross-sectional view of a surfaceacoustic wave device according to an eighth preferred embodiment of thepresent invention.

FIG. 25 is a schematic elevational cross-sectional view of a surfaceacoustic wave device according to a ninth preferred embodiment of thepresent invention.

FIG. 26 is a graph showing the relationship between the SiO₂ filmthickness and the Q_(max) factor in the case where the thickness ofpiezoelectric thin films of surface acoustic wave devices according to atenth preferred embodiment of the present invention is changed.

FIG. 27 is a schematic elevational cross-sectional view of a boundaryacoustic wave device according to an eleventh preferred embodiment ofthe present invention.

FIG. 28 is a schematic elevational cross-sectional view of a boundaryacoustic wave device according to a twelfth preferred embodiment of thepresent invention.

FIG. 29 is a schematic elevational cross-sectional view of a boundaryacoustic wave device according to a thirteenth preferred embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be clarified by describing specific preferredembodiments of the present invention with reference to the drawings.

FIG. 1A is a schematic elevational cross-sectional view of a surfaceacoustic wave device according to a first preferred embodiment of thepresent invention.

A surface acoustic wave device 1 includes a supporting substrate 2. Ahigh-acoustic-velocity film 3 having a relatively high acoustic velocityis stacked on the supporting substrate 2. A low-acoustic-velocity film 4having a relatively low acoustic velocity is stacked on thehigh-acoustic-velocity film 3. A piezoelectric film 5 is stacked on thelow-acoustic-velocity film 4. An IDT electrode 6 is stacked on the uppersurface of the piezoelectric film 5. Note that the IDT electrode 6 maybe disposed on the lower surface of the piezoelectric film 5.

The supporting substrate 2 may be composed of an appropriate material aslong as it can support the laminated structure including thehigh-acoustic-velocity film 3, the low-acoustic-velocity film 4, thepiezoelectric film 5, and the IDT electrode 6. Examples of such amaterial that can be used include piezoelectrics, such as sapphire,lithium tantalate, lithium niobate, and quartz; various ceramics, suchas alumina, magnesia, silicon nitride, aluminum nitride, siliconcarbide, zirconia, cordierite, mullite, steatite, and forsterite;dielectrics, such as glass; semiconductors, such as silicon and galliumnitride; and resin substrates. In this preferred embodiment, thesupporting substrate 2 is preferably composed of glass.

The high-acoustic-velocity film 3 functions in such a manner that asurface acoustic wave is confined to a portion in which thepiezoelectric film 5 and the low-acoustic-velocity film 4 are stackedand the surface acoustic wave does not leak into the structure below thehigh-acoustic-velocity film 3. In this preferred embodiment, thehigh-acoustic-velocity film 3 is preferably composed of aluminumnitride. As the material for high-acoustic-velocity film 3, as long asit is capable of confining the elastic wave, any of varioushigh-acoustic-velocity materials can be used. Examples thereof includealuminum nitride, aluminum oxide, silicon carbide, silicon nitride,silicon oxynitride, a DLC film or diamond, media mainly composed ofthese materials, and media mainly composed of mixtures of thesematerials. In order to confine the surface acoustic wave to the portionin which the piezoelectric film 5 and the low-acoustic-velocity film 4are stacked, it is preferable that the thickness of thehigh-acoustic-velocity film 3 be as large as possible. The thickness ofthe high-acoustic-velocity film 3 is preferably about 0.5 times or more,more preferably about 1.5 times or more, than the wavelength λ of thesurface acoustic wave.

In this description, the “high-acoustic-velocity film” is defined as afilm in which the acoustic velocity of a bulk wave propagating thereinis higher than the acoustic velocity of an elastic wave, such as asurface acoustic wave or a boundary acoustic wave, propagating in oralong the piezoelectric film 5. Furthermore, the “low-acoustic-velocityfilm” is defined as a film in which the acoustic velocity of a bulk wavepropagating therein is lower than the acoustic velocity of a bulk wavepropagating in the piezoelectric film 5. Furthermore, elastic waves withvarious modes having different acoustic velocities are excited by an IDTelectrode having a certain structure. The “elastic wave propagating inthe piezoelectric film 5” represents an elastic wave with a specificmode used for obtaining filter or resonator characteristics. The bulkwave mode that determines the acoustic velocity of the bulk wave isdefined in accordance with the usage mode of the elastic wavepropagating in the piezoelectric film 5. In the case where thehigh-acoustic-velocity film 3 and the low-acoustic-velocity film 4 areisotropic with respect to the propagation direction of the bulk wave,correspondences are as shown in Table 1 below. That is, for the dominantmode of the elastic wave shown in the left column of Table 1, the highacoustic velocity and the low acoustic velocity are determined accordingto the mode of the bulk wave shown in the right column of Table 1. The Pwave is a longitudinal wave, and the S wave is a transversal wave.

In Table 1, U1 represents an elastic wave containing as a majorcomponent a P wave, U2 represents an elastic wave containing as a majorcomponent an SH wave, and U3 represents an elastic wave containing as amajor component an SV wave.

TABLE 1 Correspondence of the elastic wave mode of the piezoelectricfilm to the bulk wave mode of the dielectric film (in the case where thedielectric film is composed of an isotropic material) Dominant mode ofthe Mode of the elastic wave propagating bulk wave propagating in thepiezoelectric film in the dielectric film U1 P wave U2 S wave U3 + U1 Swave

In the case where the low-acoustic-velocity film 4 and thehigh-acoustic-velocity film 3 are anisotropic with respect to thepropagation of the bulk wave, bulk wave modes that determine the highacoustic velocity and the low acoustic velocity are shown in Table 2below. In addition, in the bulk wave modes, the slower of the SH waveand the SV wave is referred to as a slow transversal wave, and thefaster of the two is referred to as a fast transversal wave. Which ofthe two is the slow transversal wave depends on the anisotropy of thematerial. In LiTaO₃ or LiNbO₃ cut in the vicinity of rotated Y cut, inthe bulk wave modes, the SV wave is the slow transversal wave, and theSH wave is the fast transversal wave.

TABLE 2 Correspondence of the elastic wave mode of the piezoelectricfilm to the bulk wave mode of the dielectric film (in the case where thedielectric film is composed of an anisotropic material) Dominant mode ofthe Mode of the elastic wave propagating bulk wave propagating in thepiezoelectric film in the dielectric film U1 P wave U2 SH wave U3 + U1SV wave

In this preferred embodiment, the low-acoustic-velocity film 4 ispreferably composed of silicon oxide, and the thickness thereofpreferably is about 0.35λ, where λ is the wavelength of an elastic wavedetermined by the electrode period of the IDT electrode.

As the material constituting the low-acoustic-velocity film 4, it ispossible to use any appropriate material having a bulk wave acousticvelocity that is slower than the acoustic velocity of the bulk wavepropagating in the piezoelectric film 5. Examples of such a materialthat can be used include silicon oxide, glass, silicon oxynitride,tantalum oxide, and media mainly composed of these materials, such ascompounds obtained by adding fluorine, carbon, or boron to siliconoxide.

The low-acoustic-velocity film and the high-acoustic-velocity film areeach composed of an appropriate dielectric material capable of achievinga high acoustic velocity or a low acoustic velocity that is determinedas described above.

In this preferred embodiment, the piezoelectric film 5 is preferablycomposed of 38.5° Y cut LiTaO₃, i.e., LiTaO₃ with Euler angles of (0°,128.5°, 0°), and the thickness thereof preferably is about 0.25λ, whereλ is the wavelength of a surface acoustic wave determined by theelectrode period of the IDT electrode 6. However, the piezoelectric film5 may be composed of LiTaO₃ with other cut angles, or a piezoelectricsingle crystal other than LiTaO₃.

In this preferred embodiment, the IDT electrode 6 is preferably composedof Al. However, the IDT electrode 6 may be made of any appropriate metalmaterial, such as Al, Cu, Pt, Au, Ag, Ti, Ni, Cr, Mo, W, or an alloymainly composed of any one of these metals. Furthermore, the IDTelectrode 6 may have a structure in which a plurality of metal filmscomposed of these metals or alloys are stacked.

Although schematically shown in FIG. 1A, an electrode structure shown inFIG. 1B is disposed on the piezoelectric film 5. That is, the IDTelectrode 6 and reflectors 7 and 8 arranged on both sides in the surfaceacoustic wave electrode direction of the IDT electrode 6 are disposed. Aone-port-type surface acoustic wave resonator is thus constituted.However, the electrode structure including the IDT electrode in thepresent invention is not particularly limited, and a modification ispossible such that an appropriate resonator, a ladder filter in whichresonators are combined, a longitudinally coupled filter, a lattice-typefilter, or a transversal type filter is provided.

The surface acoustic wave device 1 according to the present preferredembodiment preferably includes the high-acoustic-velocity film 3, thelow-acoustic-velocity film 4, and the piezoelectric film 5 stacked oneach other. Thereby, the Q factor can be increased. The reason for thisis as follows.

In the related art, it is known that, by disposing ahigh-acoustic-velocity film on the lower surface of a piezoelectricsubstrate, some portion of a surface acoustic wave propagates whiledistributing energy into the high-acoustic-velocity film, and therefore,the acoustic velocity of the surface acoustic wave can be increased.

In contrast, in various preferred embodiments of the present inventionof the present application, since the low-acoustic-velocity film 4 isdisposed between the high-acoustic-velocity film 3 and the piezoelectricfilm 5, the acoustic velocity of an elastic wave is decreased. Energy ofan elastic wave essentially concentrates on a low-acoustic-velocitymedium. Consequently, it is possible to enhance an effect of confiningelastic wave energy to the piezoelectric film 5 and the IDT in which theelastic wave is excited. Therefore, in accordance with this preferredembodiment, the loss can be reduced and the Q factor can be enhancedcompared with the case where the low-acoustic-velocity film 4 is notprovided. Furthermore, the high-acoustic-velocity film 3 functions suchthat an elastic wave is confined to a portion in which the piezoelectricfilm 5 and the low-acoustic-velocity film 4 are stacked and the elasticwave does not leak into the structure below the high-acoustic-velocityfilm 3. That is, in the structure of a preferred embodiment of thepresent invention, energy of an elastic wave of a specific mode used toobtain filter or resonator characteristics is distributed into theentirety of the piezoelectric film 5 and the low-acoustic-velocity film4 and partially distributed into the low-acoustic-velocity film side ofthe high-acoustic-velocity film 3, but is not distributed into thesupporting substrate 2. The mechanism of confining the elastic wave bythe high-acoustic-velocity film is similar to that in the case of a Lovewave-type surface acoustic wave, which is a non-leaky SH wave, and forexample, is described in Kenya Hashimoto; “Introduction to simulationtechnologies for surface acoustic wave devices”; Realize; pp. 90-91. Themechanism is different from the confinement mechanism in which a Braggreflector including an acoustic multilayer film is used.

In addition, in this preferred embodiment, since thelow-acoustic-velocity film 4 is preferably composed of silicon oxide,temperature characteristics can be improved. The elastic constant ofLiTaO₃ has a negative temperature characteristic, and silicon oxide hasa positive temperature characteristic. Consequently, in the surfaceacoustic wave device 1, the absolute value of TCF can be decreased. Inaddition, the specific acoustic impedance of silicon oxide is lower thanthat of LiTaO₃. Consequently, an increase in the electromechanicalcoupling coefficient, i.e., an enhancement in the band width ratio andan improvement in frequency temperature characteristics can besimultaneously achieved.

Furthermore, by adjusting the thickness of the piezoelectric film 5 andthe thickness of each of the high-acoustic-velocity film 3 and thelow-acoustic-velocity film 4, as will be described later, theelectromechanical coupling coefficient can be adjusted in a wide range.Consequently, freedom of design can be increased.

Specific experimental examples of the surface acoustic wave deviceaccording to the preferred embodiment described above will be describedbelow to demonstrate the operation and advantageous effects of thepreferred embodiment.

A surface acoustic wave device 1 according to the first preferredembodiment and surface acoustic wave devices according to first andsecond comparative examples described below were fabricated.

First Preferred Embodiment

Al electrode (thickness: 0.08λ)/38.5° Y cut LiTaO₃ thin film (thickness:0.25λ)/silicon oxide film (thickness: 0.35λ)/aluminum nitride film(1.5λ)/supporting substrate composed of glass stacked in that order fromthe top.

First comparative example: electrode composed of Al (thickness:0.08λ)/38.5° Y cut LiTaO₃ substrate stacked in that order from the top.In the first comparative example, the electrode composed of Al wasformed on the LiTaO₃ substrate with a thickness of 350 μm.

Second Comparative Example

Al electrode (thickness: 0.08λ)/38.5° Y cut LiTaO₃ film with a thicknessof 0.5λ/aluminum nitride film (thickness: 1.5λ)/supporting substratecomposed of glass stacked in that order from the top.

In each of the surface acoustic wave devices of the first preferredembodiment and the first and second comparative examples, the electrodehad a one-port-type surface acoustic wave resonator structure shown inFIG. 1B. The wavelength λ determined by the electrode period of the IDTelectrode was 2 μm. The dominant mode of the surface acoustic wavepropagating in the 38.5° Y cut LiTaO₃ is the U2 mode, and its acousticvelocity is about 3,950 m/sec. Furthermore, the acoustic velocity of thebulk wave propagating in a rotated Y cut LiTaO₃ is constant regardlessof the rotation angle (Y cut). The acoustic velocity of the SV bulk wave(slow transversal wave) is 3,367 m/sec, and the acoustic velocity of theSH bulk wave (fast transversal wave) is 4,212 m/sec. Furthermore, ineach of the first preferred embodiment and the second comparativeexample, the aluminum nitride film is an isotropic film, and theacoustic velocity of the bulk wave (S wave) in the aluminum nitride filmis 6,000 m/sec. Furthermore, the silicon oxide film as thelow-acoustic-velocity film 4 formed in the first preferred embodiment isan isotropic film, and the acoustic velocity of the bulk wave (S wave)in silicon oxide is 3,750 m/sec. Accordingly, since the dominant mode ofthe surface acoustic wave propagating the piezoelectric is the U2 mode,the following conditions are satisfied.

(1) Acoustic velocity of the bulk wave (S wave) in thehigh-acoustic-velocity film: 6,000 m/sec>Acoustic velocity of thedominant mode (U2) of the surface acoustic wave: 3,950 m/sec.

(2) Acoustic velocity of the bulk wave (S wave) in thelow-acoustic-velocity film: 3,750 m/sec<Acoustic velocity of the bulkwave (SH) propagating in the piezoelectric film: 4,212 m/sec.

FIG. 2 shows the impedance-frequency characteristics of the surfaceacoustic wave devices of the first preferred embodiment and the firstand second comparative examples, and FIG. 3 shows an impedance Smithchart.

Furthermore, as shown in Table 3 below, in the surface acoustic wavedevices of the first preferred embodiment and the first and secondcomparative examples, the Q factor at the resonant frequency, the Qfactor at the antiresonant frequency, the band width ratio, and the TCFat the resonant frequency were obtained by actual measurement.

The results are shown in Table 3 below.

TABLE 3 Q Q (Anti- Band width TCF [ppm/° C.] (Resonance) resonance)ratio [%] (Resonance) First 818 527 3.2 −45 comparative example Second777 1285 4.1 −45 comparative example First 1026 2080 4.4 −25 embodiment

In FIGS. 2 and 3, the solid line represents the results of the firstpreferred embodiment, the dashed line represents the results of thesecond comparative example, and the dotted-chain line represents theresults of the first comparative example.

As is clear from FIGS. 2 and 3, in the second comparative example andthe first preferred embodiment, the top-to-valley ratio is higher thanthat in the first comparative example. The top-to-valley ratio is aratio of the impedance at an antiresonant frequency to the impedance ata resonant frequency. As this value increases, it becomes possible toconfigure a filter having a higher Q factor and lower insertion loss. Itis evident that, in particular, in the first preferred embodiment, thetop-to-valley ratio is much higher than that in the second comparativeexample. Furthermore, it is also evident that according to the firstpreferred embodiment, the frequency difference between the resonantfrequency and the antiresonant frequency, i.e., the band width ratio,can be increased compared with the second comparative example.

Specifically, as is clear from Table 3, according to the first preferredembodiment, the Q factor at the resonant frequency can be increased, andin particular, the Q factor at the antiresonant frequency can be greatlyincreased compared with the first and second comparative examples. Thatis, since it is possible to configure a one-port-type surface acousticwave resonator having a high Q factor, a filter having low insertionloss can be configured using the surface acoustic wave device 1.Furthermore, the band width ratio is 3.2% in the first comparativeexample and 4.1% in the second comparative example. In contrast, theband width ratio increases to 4.4% in the first preferred embodiment.

In addition, as is clear from Table 3, according to the first preferredembodiment, since the silicon oxide film is disposed, the absolute valueof TCF can be greatly decreased compared with the first and secondcomparative examples.

FIGS. 5 and 6 show the results of FEM simulation, in which thedotted-chain line represents the first preferred embodiment, the dashedline represents the first comparative example, and the solid linerepresents the second comparative example. In the FEM simulation, aone-port resonator is assumed, in which duty=0.5, the intersecting widthis 20λ, and the number of pairs is 100.

As in the experimental results described above, in the FEM simulationresults, as is clear from FIG. 6, the Q factor can also be increasedcompared with the first and second comparative examples.

Consequently, as is clear from the experimental results and the FEMsimulation results regarding the first preferred embodiment and thefirst and second comparative examples, it has been confirmed that, bydisposing the low-acoustic-velocity film composed of silicon oxidebetween the high-acoustic-velocity film 3 composed of aluminum nitrideand the piezoelectric film 5 composed of LiTaO₃, the Q factor can beenhanced. The reason for the fact that the Q factor can be enhanced isbelieved to be that energy of surface acoustic waves can be effectivelyconfined to the piezoelectric film 5, the low-acoustic-velocity film 4,and the high-acoustic-velocity film 3 by the formation of thehigh-acoustic-velocity film 3, and that the effect of suppressingleakage of energy of surface acoustic waves outside the IDT electrodecan be enhanced by the formation of the low-acoustic-velocity film 4.

Consequently, since the effect is obtained by disposing thelow-acoustic-velocity film 4 between the piezoelectric film 5 and thehigh-acoustic-velocity film 3 as described above, the materialconstituting the piezoelectric film is not limited to the 38.5° Y cutLiTaO₃ described above. The same effect can be obtained in the casewhere LiTaO₃ with other cut angles is used. Furthermore, the same effectcan be obtained in the case where a piezoelectric single crystal such asLiNbO₃ other than LiTaO₃, a piezoelectric thin film such as ZnO or AlN,or a piezoelectric ceramic such as PZT is used.

Furthermore, the high-acoustic-velocity film 3 has a function ofconfining the majority of energy of surface acoustic waves to a portionin which the piezoelectric film 5 and the low-acoustic-velocity film 4are stacked. Consequently, the aluminum nitride film may be ac-axis-oriented, anisotropic film. Furthermore, the material for thehigh-acoustic-velocity film 3 is not limited to the aluminum nitridefilm, and it is expected that the same effect can be obtained in thecase where any of various materials that can constitute thehigh-acoustic-velocity film 3 described above is used.

Furthermore, silicon oxide of the low-acoustic-velocity film is notparticularly limited as long as the acoustic velocity of a bulk wavepropagating therein is lower than the acoustic velocity of a bulk wavepropagating in the piezoelectric film. Consequently, the materialconstituting the low-acoustic-velocity film 4 is not limited to siliconoxide. Therefore, any of the various materials described above asexamples of a material that can constitute the low-acoustic-velocityfilm 4 can be used.

Second Preferred Embodiment

Characteristics of a surface acoustic wave device according to a secondpreferred embodiment having the structure described below were simulatedby a finite element method. The electrode structure was the same as thatshown in FIG. 1B.

An IDT electrode was an Al film with a thickness of 0.08λ. Apiezoelectric film was composed of 38.5° Y cut LiTaO₃ film, and thethickness thereof was in a range of 0 to 3λ. A low-acoustic-velocityfilm was composed of silicon oxide, and the thickness thereof was 0 to2λ. A high-acoustic-velocity film was composed of aluminum oxide, andthe thickness thereof was 1.5λ. A supporting substrate was composed ofalumina.

The results are shown in FIGS. 7 to 10.

FIG. 7 is a graph showing the relationship between the LiTaO₃ filmthickness, the acoustic velocity of the U2 mode which is the usage mode,and the normalized film thickness of the silicon oxide film.Furthermore, FIG. 8 is a graph showing the relationship between theLiTaO₃ film thickness, the electromechanical coupling coefficient k² ofthe U2 mode which is the usage mode, and the normalized film thicknessof the silicon oxide film.

As is clear from FIG. 7, by forming the silicon oxide film, thevariations in acoustic velocity are small in the wide thickness range of0.05λ to 0.5λ of the piezoelectric film composed of LiTaO₃, incomparison with the case where the normalized film thickness of thesilicon oxide film is 0.0, i.e., no silicon oxide film is formed.

Furthermore, as is clear from FIG. 8, when the silicon oxide film isformed, even in the case where the LiTaO₃ film thickness is small at0.35λ or less, by controlling the silicon oxide film thickness, theelectromechanical coupling coefficient k² can be increased to 0.08 ormore, in comparison with the case where no silicon oxide film is formed.

FIG. 9 is a graph showing the relationship between the LiTaO₃ filmthickness, the temperature coefficient of frequency TCV, and thenormalized film thickness of the silicon oxide film. FIG. 10 is a graphshowing the relationship between the LiTaO₃ film thickness, the bandwidth ratio, and the normalized film thickness of the silicon oxidefilm.

Note that TCF=TCV−α, where α is the coefficient of linear expansion inthe propagation direction. In the case of LiTaO₃, α is about 16 ppm/° C.

As is clear from FIG. 9, by forming the silicon oxide film, the absolutevalue of TCV can be further decreased in comparison with the case whereno silicon oxide film is formed. In addition, as is clear from FIG. 10,even in the case where the thickness of the piezoelectric film composedof LiTaO₃ is small at about 0.35λ or less, by adjusting the siliconoxide film thickness, the band width ratio can be adjusted.

Furthermore, when the thickness of the silicon oxide film is increasedto more than about 2λ, stress is generated, resulting in problems, suchas warpage of the surface acoustic wave device, which may cause handlingdifficulty. Consequently, the thickness of the silicon oxide film ispreferably about 2λ or less.

In the related art, it is known that, by using a laminated structure inwhich an IDT is disposed on LiTaO₃ and silicon oxide is further disposedon the IDT, the absolute value of TCF in the surface acoustic wavedevice can be decreased. However, as is clear from FIG. 11, when theabsolute value of TCV is intended to be decreased, i.e., when theabsolute value of TCF is intended to be decreased, it is not possible tosimultaneously achieve an increase in the bandwidth ratio and a decreasein the absolute value of the TCF. In contrast, by using the structure ofthe present invention in which the high-acoustic-velocity film and thelow-acoustic-velocity film are stacked, a decrease in the absolute valueof TCF and an increase in the band width ratio can be achieved. Thiswill be described with reference to FIGS. 11 and 12.

FIG. 11 is a graph showing the relationship between the band width ratioand the TCF in surface acoustic wave devices of third to fifthcomparative examples described below as conventional surface acousticwave devices.

Third comparative example: laminated structure of electrode composed ofAl/42° Y cut LiTaO₃. SH wave was used.

Fourth comparative example: laminated structure of silicon oxidefilm/electrode composed of Cu/38.5° Y cut LiTaO₃ substrate. SH wave wasused.

Fifth comparative example: laminated structure of silicon oxidefilm/electrode composed of Cu/128° Y cut LiNbO₃ substrate. SV wave wasused.

As is clear from FIG. 11, in any of the third comparative example to thefifth comparative example, as the band width ratio BW increases, theabsolute value of TCF increases.

FIG. 12 is a graph showing the relationship between the band width ratioBW (%) and the temperature coefficient of frequency TCV in the casewhere the normalized film thickness of LiTaO₃ was changed in the rangeof about 0.1λ to about 0.5λ in each of the thickness levels of thesilicon oxide film of the second preferred embodiment. As is clear fromFIG. 12, in this preferred embodiment, even in the case where the bandwidth ratio BW is increased, the absolute value of TCV does notincrease. That is, by adjusting the thickness of the silicon oxide film,the band width ratio can be increased, and the absolute value of thetemperature coefficient of frequency TCV can be decreased.

That is, by stacking the low-acoustic-velocity film 4 and thehigh-acoustic-velocity film 3 on the piezoelectric film composed ofLiTaO₃, and in particular, by forming a silicon oxide film as thelow-acoustic-velocity film, it is possible to provide an elastic wavedevice having a wide band width ratio and good temperaturecharacteristics.

Preferably, the coefficient of linear expansion of the supportingsubstrate 2 is smaller than that of the piezoelectric film 5. As aresult, expansion due to heat generated in the piezoelectric film 5 isrestrained by the supporting substrate 2. Consequently, the frequencytemperature characteristics of the elastic wave device can be furtherimproved.

FIGS. 13 and 14 are graphs showing changes in the acoustic velocity andchanges in the band width ratio, respectively, with changes in thethickness of the piezoelectric film composed of LiTaO₃ in the structureof the second preferred embodiment.

As is clear from FIGS. 13 and 14, when the LiTaO₃ thickness is about1.5λ or more, the acoustic velocity and the band width ratio are nearlyunchanged. The reason for this is that energy of surface acoustic wavesis confined to the piezoelectric film and is not distributed into thelow-acoustic-velocity film 4 and the high-acoustic-velocity film 3.Consequently, the effects of the low-acoustic-velocity film 4 and thehigh-acoustic-velocity film 3 are not exhibited. Therefore, it is morepreferable to set the thickness of the piezoelectric film to be about1.5λ or less. Thereby, it is believed that energy of surface acousticwaves can be sufficiently distributed into the low-acoustic-velocityfilm 4 and the Q factor can be further enhanced.

The results of FIGS. 7 to 14 show that, by adjusting the thickness ofthe silicon oxide film and the thickness of the piezoelectric filmcomposed of LiTaO₃, the electromechanical coupling coefficient can beadjusted over a wide range. Furthermore, it is clear that when thethickness of the piezoelectric film composed of LiTaO₃ is in the rangeof about 0.05λ to about 0.5λ, the electromechanical coupling coefficientcan be adjusted in a wider range. Consequently, the thickness of thepiezoelectric film composed of LiTaO₃ is preferably in the range ofabout 0.05λ to about 0.5λ.

Conventionally, it has been required to adjust cut angles of thepiezoelectric used in order to adjust the electromechanical couplingcoefficient. However, when the cut angles, i.e., Euler angles, arechanged, other material characteristics, such as the acoustic velocity,temperature characteristics, and spurious characteristics, are alsochanged. Consequently, it has been difficult to simultaneously satisfythese characteristics, and optimization of design has been difficult.

However, as is clear from the results of the second preferred embodimentdescribed above, according to the present invention, even in the casewhere a piezoelectric single crystal with the same cut angles is used asthe piezoelectric film, by adjusting the thickness of the silicon oxidefilm, i.e., the low-acoustic-velocity film, and the thickness of thepiezoelectric film, the electromechanical coupling coefficient can befreely adjusted. Consequently, freedom of design can be greatlyincreased. Therefore, it is enabled to simultaneously satisfy variouscharacteristics, such as the acoustic velocity, the electromechanicalcoupling coefficient, frequency temperature characteristics, andspurious characteristics, and it is possible to easily provide a surfaceacoustic wave device having desired characteristics.

Third Preferred Embodiment

As a third preferred embodiment, surface acoustic wave devices same asthose of the first preferred embodiment were fabricated. The materialsand thickness were as described below.

A laminated structure included an Al film with a thickness of 0.08λ asan IDT electrode 6/a LiTaO₃ film with a thickness of 0.25λ as apiezoelectric film 4/a silicon oxide film with a thickness in the rangeof 0 to 2λ as a low-acoustic-velocity film 4/a high-acoustic-velocityfilm. As the high-acoustic-velocity film, a silicon nitride film, analuminum oxide film, or diamond was used. The thickness of thehigh-acoustic-velocity film 3 was 1.5λ.

FIGS. 15 and 16 are graphs showing the relationship between thethickness of the silicon oxide film and the acoustic velocity and therelationship between the thickness of the silicon oxide film and theelectromechanical coupling coefficient k², respectively, in the thirdpreferred embodiment.

The acoustic velocity of the bulk wave (S wave) in the silicon nitridefilm is 6,000 m/sec, and the acoustic velocity of the bulk wave (S wave)in aluminum oxide is 6,000 m/sec. Furthermore, the acoustic velocity ofthe bulk wave (S wave) in diamond is 12,800 m/sec.

As is clear from FIGS. 15 and 16, as long as the high-acoustic-velocityfilm 4 satisfies the conditions for the high-acoustic-velocity film 4described earlier, even if the material for the high-acoustic-velocityfilm 4 and the thickness of the silicon oxide film are changed, theelectromechanical coupling coefficient and the acoustic velocity arenearly unchanged. In particular, if the thickness of the silicon oxidefilm is about 0.1λ or more, the electromechanical coupling coefficientis nearly unchanged in the silicon oxide film thickness range of about0.1λ to about 0.5λ regardless of the material for thehigh-acoustic-velocity film. Furthermore, as is clear from FIG. 15, inthe silicon oxide film thickness range of about 0.3λ to about 2λ, theacoustic velocity is nearly unchanged regardless of the material for thehigh-acoustic-velocity film.

Consequently, in the present invention, the material for thehigh-acoustic-velocity film is not particularly limited as long as theabove conditions are satisfied.

Fourth Preferred Embodiment

In a fourth preferred embodiment, while changing the Euler angles (0°,θ, ψ) of the piezoelectric film, the electromechanical couplingcoefficient of a surface acoustic wave containing as a major componentthe U2 component (SH component) was measured.

A laminated structure was composed of IDT electrode 6/piezoelectric film5/low-acoustic-velocity film 4/high-acoustic-velocity film 3/supportingsubstrate 2. As the IDT electrode 6, Al with a thickness of 0.08λ wasused. As the piezoelectric film, LiTaO₃ with a thickness of 0.25λ wasused. As the low-acoustic-velocity film 4, silicon oxide with athickness of 0.35λ was used. As the high-acoustic-velocity film 3, analuminum nitride film with a thickness of 1.5λ was used. As thesupporting substrate 2, glass was used.

In the structure described above, regarding many surface acoustic wavedevices with Euler angles (0°, θ, ψ) in which θ and ψ were varied, theelectromechanical coupling coefficient was obtained by FEM. As a result,it was confirmed that in a plurality of regions R1 shown in FIG. 17, theelectromechanical coupling coefficient k² of the mode mainly composed ofthe U2 component (SH component) is about 2% or more. Note that the sameresults were obtained in the range of Euler angles (0°±5, θ, ψ).

That is, when LiTaO₃ with Euler angles located in a plurality of rangesR1 shown in FIG. 17 is used, the electromechanical coupling coefficientof the vibration mainly composed of the U2 component is about 2% ormore. Therefore, it is clear that a bandpass filter with a wide bandwidth can be configured using a surface acoustic wave device accordingto a preferred embodiment of the present invention.

Fifth Preferred Embodiment

Assuming the same structure as that in the fourth preferred embodiment,the electromechanical coupling coefficient of a surface acoustic wavemainly composed of the U3 component (SV component) was obtained by FEM.The range of Euler angles in which the electromechanical couplingcoefficient of the mode mainly composed of the U2 (SH component) isabout 2% or more, and the electromechanical coupling coefficient of themode mainly composed of the U3 (SV component) is about 1% or less wasobtained. The results are shown in FIG. 18. In a plurality of ranges R2shown in FIG. 18, the electromechanical coupling coefficient of the modemainly composed of the U2 (SH component) is about 2% or more, and theelectromechanical coupling coefficient of the mode mainly composed ofthe U3 (SV component) is about 1% or less. Consequently, by using LiTaO₃with Euler angles located in any one of a plurality of regions R2, theelectromechanical coupling coefficient of the U2 mode used can beincreased and the electromechanical coupling coefficient of the U3 modewhich is spurious can be decreased. Therefore, it is possible toconfigure a bandpass filter having better filter characteristics.

Sixth Preferred Embodiment

As in the second preferred embodiment, simulation was carried out on asurface acoustic wave device having the structure described below. Asshown in Table 4 below, in the case where the transversal wave acousticvelocity of the low-acoustic-velocity film and the specific acousticimpedance of the transversal wave of the low-acoustic-velocity film werechanged in 10 levels, characteristics of surface acoustic waves mainlycomposed of the U2 component were simulated by a finite element method.In the transversal wave acoustic velocity and specific acousticimpedance of the low-acoustic-velocity film, the density and elasticconstant of the low-acoustic-velocity film were changed. Furthermore, asthe material constants of the low-acoustic-velocity film not shown inTable 4, material constants of silicon oxide were used.

TABLE 4 Specific acoustic impedance of Specific Elastic constantTransversal wave transversal wave gravity ρ C11 C44 acoustic velocity VZs Level [kg/m³] [N/m²] [N/m²] [m/s] [N · s/m³] Remarks 1 1.11E+034.73E+10 1.56E+10 3757 4.2.E+06 2 2.21E+03 7.85E+10 3.12E+10 37578.3.E+06 Silicon oxide equivalent 3 3.32E+03 1.10E+11 4.68E+10 37571.2.E+07 4 6.63E+03 2.03E+11 9.36E+10 3757 2.5.E+07 5 1.11E+04 3.28E+111.56E+11 3757 4.2.E+07 6 2.21E+03 3.17E+10 7.80E+09 1879 4.2.E+06 74.42E+03 4.73E+10 1.56E+10 1879 8.3.E+06 8 6.63E+03 6.29E+10 2.34E+101879 1.2.E+07 9 1.33E+04 1.10E+11 4.68E+10 1879 2.5.E+07 10 2.21E+041.72E+11 7.80E+10 1879 4.2.E+07

Note that, in Table 4, 1.11E+03 means 1.11×10³. That is, aE+b representsa×10^(b).

The electrode structure was the same as that shown in FIG. 1B, and thesurface acoustic wave device had a laminated structure of IDTelectrode/piezoelectric film/low-acoustic-velocityfilm/high-acoustic-velocity film/supporting substrate. The IDT electrodewas an Al film with a thickness of 0.08λ. The piezoelectric film wascomposed of 40° Y cut LiTaO₃. In each of the cases where the thicknessof the piezoelectric film was 0.1λ, 0.4λ, and 0.6λ, 10 levels shown inTable 4 were calculated. The thickness of the low-acoustic-velocity filmwas 0.4λ. The high-acoustic-velocity film was composed of aluminumoxide, and the thickness thereof was 1.5λ. The supporting substrate wascomposed of an alumina substrate.

FIGS. 19A to 19C are graphs showing the relationships between thespecific acoustic impedance of the low-acoustic-velocity film and theband width ratio in the sixth preferred embodiment. In the graphs, eachlevel shows the behavior in the case where the acoustic velocity of thetransversal wave in the low-acoustic-velocity film changes, and the bandwidth ratio in each level is normalized to the band width ratio in thecase where the specific acoustic impedance of the piezoelectric film isequal to the specific acoustic impedance of the low-acoustic-velocityfilm. The specific acoustic impedance is expressed as a product of theacoustic velocity of the bulk wave and the density of the medium. In thesixth preferred embodiment, the bulk wave of the piezoelectric film isthe SH bulk wave, the acoustic velocity is 4,212 m/s, and the density is7.454×10³ kg/m³. Consequently, the specific acoustic impedance of thepiezoelectric film is 3.14×10⁷ N·s/m³. Furthermore, regarding theacoustic velocity of the bulk wave used for calculating the specificacoustic impedance of each of the low-acoustic-velocity film and thepiezoelectric film, for the dominant mode of the elastic wave shown inthe left column of Table 1 or 2, the acoustic velocity is determinedaccording to the mode of the bulk wave shown in the right column ofTable 1 or 2.

Furthermore, FIGS. 20A to 20C are graphs showing the relationshipsbetween the specific acoustic impedance of the transversal wave of thelow-acoustic-velocity film and the acoustic velocity of the propagatingsurface acoustic wave in the sixth preferred embodiment.

As is clear from FIGS. 19A to 19C, regardless of the thickness of thepiezoelectric film, the band width ratio increases as the specificacoustic impedance of the low-acoustic-velocity film becomes smallerthan the specific acoustic impedance of the piezoelectric film. Thereason for this is that since the specific acoustic impedance of thelow-acoustic-velocity film is smaller than the specific acousticimpedance of the piezoelectric film, the displacement of thepiezoelectric film under certain stress increases, thus generating alarger electric charge, and therefore, equivalently higherpiezoelectricity can be obtained. That is, since this effect is obtaineddepending only on the magnitude of specific acoustic impedance,regardless of the vibration mode of the surface acoustic wave, the typeof the piezoelectric film, or the type of the low-acoustic-velocityfilm, it is possible to obtain a surface acoustic wave device having ahigher band width ratio when the specific acoustic impedance of thelow-acoustic-velocity film is smaller than the specific impedance of thepiezoelectric film.

In each of the first to sixth preferred embodiments of the presentinvention, the IDT electrode 6, the piezoelectric film 5, thelow-acoustic-velocity film 4, the high-acoustic-velocity film 3, and thesupporting substrate 2 preferably are stacked in that order from thetop, for example. However, within the extent that does not greatlyaffect the propagating surface acoustic wave and boundary wave, anadhesion layer composed of Ti, NiCr, or the like, an underlying film, orany medium may be disposed between the individual layers. In such acase, the same effect can be obtained. For example, a newhigh-acoustic-velocity film which is sufficiently thin compared with thewavelength of the surface acoustic wave may be disposed between thepiezoelectric film 5 and the low-acoustic-velocity film 4. In such acase, the same effect can be obtained. Furthermore, energy of the mainlyused surface acoustic wave is not distributed between thehigh-acoustic-velocity film 3 and the supporting substrate 2.Consequently, any medium with any thickness may be disposed between thehigh-acoustic-velocity film and the supporting substrate 2. In such acase, the same advantageous effects can be obtained.

The seventh and eighth preferred embodiments described below relate tosurface acoustic wave devices provided with such a medium layer.

Seventh Preferred Embodiment

In a surface acoustic wave device 21 according to a seventh preferredembodiment shown in FIG. 23, a medium layer 22 is disposed between asupporting substrate 2 and a high-acoustic-velocity film 3. Thestructure other than this is the same as that in the first preferredembodiment. Therefore, the description of the first preferred embodimentis incorporated herein. In the surface acoustic wave device 21, an IDTelectrode 6, a piezoelectric film 5, a low-acoustic-velocity film 4, thehigh-acoustic-velocity film 3, the medium layer 22, and the supportingsubstrate 2 are stacked in that order from the top.

As the medium layer 22, any material, such as a dielectric, apiezoelectric, a semiconductor, or a metal, may be used. Even in such acase, the same effect as that of the first preferred embodiment can beobtained. In the case where the medium layer 22 is composed of a metal,the band width ratio can be decreased. Consequently, in the applicationin which the band width ratio is small, the medium layer 22 ispreferably composed of a metal.

Eighth Preferred Embodiment

In a surface acoustic wave device 23 according to an eighth preferredembodiment shown in FIG. 24, a medium layer 22 and a medium layer 24 aredisposed between a supporting substrate 2 and a high-acoustic-velocityfilm 3. That is, an IDT electrode 6, a piezoelectric film 5, alow-acoustic-velocity film 4, the high-acoustic-velocity film 3, themedium layer 22, the medium layer 24, and the supporting substrate 2 arestacked in that order from the top. Other than the medium layer 22 andthe medium layer 24, the structure is the same as that in the firstpreferred embodiment.

The medium layers 22 and 24 may be composed of any material, such as adielectric, a piezoelectric, a semiconductor, or a metal. Even in such acase, in the eighth preferred embodiment, it is possible to obtain thesame effect as that of the surface acoustic wave device of the firstpreferred embodiment.

In this preferred embodiment, after a laminated structure including thepiezoelectric film 5, the low-acoustic-velocity film 4, thehigh-acoustic-velocity film 3, and the medium layer 22 and a laminatedstructure including the medium layer 24 and the supporting substrate 2are separately fabricated, both laminated structures are bonded to eachother. Then, the IDT electrode 6 is formed on the piezoelectric film 5.As a result, it is possible to obtain a surface acoustic wave deviceaccording to this preferred embodiment without being restricted bymanufacturing conditions when each laminated structure is fabricated.Consequently, freedom of selection for materials constituting theindividual layers can be increased.

When the two laminated structures are bonded to each other, any joiningmethod can be used. For such a bonded structure, various methods, suchas bonding by hydrophilization, activation bonding, atomic diffusionbonding, metal diffusion bonding, anodic bonding, bonding using a resinor SOG, can be used. Furthermore, the joint interface between the twolaminated structures is located on the side opposite to thepiezoelectric film 5 side of the high-acoustic-velocity film 3.Consequently, the joint interface exists in the portion below thehigh-acoustic-velocity film 3 in which major energy of the surfaceacoustic wave used is not distributed. Therefore, surface acoustic wavepropagation characteristics are not affected by the quality of the jointinterface. Accordingly, it is possible to obtain stable and goodresonance characteristics and filter characteristics.

Ninth Preferred Embodiment

In a surface acoustic wave device 31 shown in FIG. 25, an IDT electrode6, a piezoelectric film 5, a low-acoustic-velocity film 4, and ahigh-acoustic-velocity supporting substrate 33 which also functions as ahigh-acoustic-velocity film are stacked in that order from the top. Thatis, the high-acoustic-velocity supporting substrate 33 serves both asthe high-acoustic-velocity film 3 and as the supporting substrate 2 inthe first preferred embodiment. Consequently, the acoustic velocity of abulk wave in the high-acoustic-velocity supporting substrate 33 is setto be higher than the acoustic velocity of a surface acoustic wavepropagating in the piezoelectric film 5. Thus, the same effect as thatin the first preferred embodiment can be obtained. Moreover, since thehigh-acoustic-velocity supporting substrate 33 serves both as thehigh-acoustic-velocity film and as the supporting substrate, the numberof components can be reduced.

Tenth Preferred Embodiment

In a tenth preferred embodiment, the relationship between the Q factorand the frequency in a one-port-type surface acoustic wave resonator asa surface acoustic wave device was simulated by FEM.

Here, as the surface acoustic wave device according to the firstpreferred embodiment, shown in FIGS. 1A and 1B, the following structurewas assumed.

The structure included an IDT electrode 6 composed of Al with athickness of 0.1λ, a piezoelectric film composed of a 50° Y cut LiTaO₃film, an SiO₂ film as a low-acoustic-velocity film, an aluminum nitridefilm with a thickness of 1.5λ as a high-acoustic-velocity film, an SiO₂film with a thickness of 0.3λ, and a supporting substrate composed ofalumina stacked in that order from the top. In this simulation, thethickness of the LiTaO₃ film as the piezoelectric film was changed to0.15λ, 0.20λ, 0.25λ, or 0.30λ. Furthermore, the thickness of the SiO₂film as the low-acoustic-velocity film was changed in the range of 0 to2λ.

The duty of the IDT electrode was 0.5, the intersecting width ofelectrode fingers was 20λ, and the number of electrode finger pairs was100.

For comparison, a one-port-type surface acoustic wave resonator, inwhich an IDT electrode composed of Al with a thickness of 0.1λ and a38.5° Y cut LiTaO₃ substrate were stacked in that order from the top,was prepared. That is, in the comparative example, an electrodestructure including the IDT electrode composed of Al is disposed on a38.5° Y cut LiTaO₃ substrate with a thickness of 350 μm.

Regarding the surface acoustic wave devices according to the tenthpreferred embodiment and the comparative example, the relationshipbetween the Q factor and the frequency was obtained by simulation byFEM. In the range from the resonant frequency at which the impedance ofthe one-port resonator was lowest to the antiresonant frequency at whichthe impedance was highest, the highest Q factor was defined as theQ_(max) factor. A higher Q_(max) factor indicates lower loss.

The Q_(max) factor of the comparative example was 857. FIG. 26 shows therelationship between the LiTaO₃ film thickness, the SiO₂ film thickness,and the Q_(max) in this preferred embodiment.

As is clear from FIG. 26, in each case where the LiTaO₃ film thicknessis 0.15λ, 0.20λ, 0.25λ, or 0.30λ, the Q_(max) factor increases when thethickness of the low-acoustic-velocity film composed of SiO₂ exceeds 0.It is also clear that in the tenth preferred embodiment, in any of thecases, the Q_(max) factor is effectively enhanced relative to thecomparative example.

Preferred Embodiment of Manufacturing Method

The elastic wave device according to the first preferred embodimentincludes, as described above, the high-acoustic-velocity film 3, thelow-acoustic-velocity film 4, the piezoelectric film 5, and the IDTelectrode 6 which are disposed on the supporting substrate 2. The methodfor manufacturing such an elastic wave device is not particularlylimited. By using a manufacturing method using the ion implantationprocess described below, it is possible to easily obtain an elastic wavedevice 1 having a piezoelectric film with a small thickness. A preferredembodiment of the manufacturing method will be described with referenceto FIGS. 21A-21E and 22A-22C.

First, as shown in FIG. 21A, a piezoelectric substrate 5A is prepared.In this preferred embodiment, the piezoelectric substrate 5A ispreferably composed of LiTaO₃. Hydrogen ions are implanted from asurface of the piezoelectric substrate 5A. The ions to be implanted arenot limited to hydrogen, and helium or the like may be used.

In the ion implantation, energy is not particularly limited. In thispreferred embodiment, preferably the energy is about 107 KeV, and thedose amount is about 8×10¹⁶ atoms/cm², for example.

When ion implantation is performed, the ion concentration is distributedin the thickness direction in the piezoelectric substrate 5A. In FIG.21A, the dashed line represents a region in which the ion concentrationis highest. In a high concentration ion-implanted region 5 a in whichthe ion concentration is highest represented by the dashed line, whenheating is performed as will be described later, separation easilyoccurs owing to stress. Such a method in which separation is performedusing the high concentration ion-implanted region 5 a is disclosed inJapanese Unexamined Patent Application Publication No. 2002-534886.

In this step, at the high concentration ion-implanted region 5 a, thepiezoelectric substrate 5A is separated to obtain a piezoelectric film5. The piezoelectric film 5 is a layer between the high concentrationion-implanted region 5 a and the surface of the piezoelectric substratefrom which ion implantation is performed. In some cases, thepiezoelectric film 5 may be subjected to machining, such as grinding.Consequently, the distance from the high concentration ion-implantedregion 5 a to the surface of the piezoelectric substrate on the ionimplantation side is set to be equal to or slightly larger than thethickness of the finally formed piezoelectric film.

Next, as shown in FIG. 21B, a low-acoustic-velocity film 4 is formed onthe surface of the piezoelectric substrate 5A on which the ionimplantation has been performed. In addition, a low-acoustic-velocityfilm formed in advance may be bonded to the piezoelectric substrate 5Ausing a transfer method or the like.

Next, as shown in FIG. 21C, a high-acoustic-velocity film 3 is formed ona surface of the low-acoustic-velocity film 4, opposite to thepiezoelectric substrate 5A side of the low-acoustic-velocity film 4.Instead of using the film formation method, the high-acoustic-velocityfilm 3 may also be bonded to the low-acoustic-velocity film 4 using atransfer method or the like.

Furthermore, as shown in FIG. 21D, an exposed surface of thehigh-acoustic-velocity film 3, opposite to the low-acoustic-velocityfilm 4 side of the high-acoustic-velocity film 3, is subjected to mirrorfinishing. By performing mirror finishing, it is possible to strengthenbonding between the high-acoustic-velocity film and the supportingsubstrate which will be described later.

Then, as shown in FIG. 21E, a supporting substrate 2 is bonded to thehigh-acoustic-velocity film 3.

As the low-acoustic-velocity film 4, as in the first preferredembodiment, a silicon oxide film is used. As the high-acoustic-velocityfilm 3, an aluminum nitride film is used.

Next, as shown in FIG. 22A, after heating, a piezoelectric substrateportion 5 b located above the high concentration ion-implanted region 5a in the piezoelectric substrate 5A is separated. As described above, byapplying stress by heating through the high concentration ion-implantedregion 5 a, the piezoelectric substrate 5A becomes easily separated. Inthis case, the heating temperature may be set at about 250° C. to 400°C., for example.

In this preferred embodiment, by the heat-separation, a piezoelectricfilm 5 with a thickness of about 500 nm, for example, is obtained. Insuch a manner, as shown in FIG. 22B, a structure in which thepiezoelectric film 5 is stacked on the low-acoustic-velocity film 4 isobtained. Then, in order to recover piezoelectricity, heat treatment isperformed in which the structure is retained at a temperature of about400° C. to about 500° C. for about 3 hours, for example. Optionally,prior to the heat treatment, the upper surface of the piezoelectric film5 after the separation may be subjected to grinding.

Then, as shown in FIG. 22C, an electrode including an IDT electrode 6 isformed. The electrode formation method is not particularly limited, andan appropriate method, such as vapor deposition, plating, or sputtering,may be used, for example.

According to the manufacturing method of this preferred embodiment, bythe separation, it is possible to easily form a piezoelectric film 5with rotated Euler angles at a uniform thickness.

Eleventh Preferred Embodiment

In the first preferred embodiment, the IDT electrode 6, thepiezoelectric film 5, the low-acoustic-velocity film 4, thehigh-acoustic-velocity film 3, and the supporting substrate 2 arepreferably stacked in that order from the top. In a surface acousticwave device 41 according to an eleventh preferred embodiment shown inFIG. 27, a dielectric film 42 may be arranged so as to cover an IDTelectrode 6. By disposing such a dielectric film 42, frequencytemperature characteristics can be adjusted, and moisture resistance canbe enhanced.

Twelfth Preferred Embodiment

In the preferred embodiments described above, description has beenprovided for surface acoustic wave devices. The present invention canalso be applied to other elastic wave devices, such as boundary acousticwave devices. In such a case, the same advantageous effects can also beobtained. FIG. 28 is a schematic elevational cross-sectional view of aboundary acoustic wave device 43 according to a twelfth preferredembodiment. In this case, a low-acoustic-velocity film 4, ahigh-acoustic-velocity film 3, and a supporting substrate 2 arepreferably stacked in that order from the top, under a piezoelectricfilm 5. This structure is preferably the same or substantially the sameas that of the first preferred embodiment. In order to excite a boundaryacoustic wave, an IDT electrode 6 is provided at the interface betweenthe piezoelectric film 5 and a dielectric 44 disposed on thepiezoelectric film 5.

Furthermore, FIG. 29 is a schematic elevational cross-sectional view ofa boundary acoustic wave device 45 having a three-medium structure. Inthis case, with respect to a structure in which a low-acoustic-velocityfilm 4, a high-acoustic-velocity film 3, and a supporting substrate 2are stacked in that order, under a piezoelectric film 5, an IDTelectrode 6 is provided at the interface between the piezoelectric film5 and a dielectric film 46. Furthermore, a dielectric 47 in which theacoustic velocity of a transversal wave propagating therein is fasterthan that of the dielectric 46 is disposed on the dielectric 46. As aresult, a boundary acoustic wave having a three-medium structure isprovided.

In the boundary acoustic wave device, such as the boundary acoustic wavedevice 43 or 45, as in the surface acoustic wave device 1 according tothe first preferred embodiment, by disposing a laminated structurecomposed of low-acoustic-velocity film 4/high-acoustic-velocity film 3on the lower side of the piezoelectric film 5, the same effect as thatin the first preferred embodiment can be obtained.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

What is claimed is:
 1. An elastic wave device including a piezoelectricfilm, the elastic wave device comprising: a high-acoustic-velocitysupporting substrate in which an acoustic velocity of a bulk wavepropagating therein is higher than an acoustic velocity of an elasticwave propagating in the piezoelectric film; a low-acoustic-velocity filmstacked on the high-acoustic-velocity supporting substrate; thepiezoelectric film stacked on the low-acoustic-velocity film; and an IDTelectrode disposed on a surface of the piezoelectric film; wherein athickness of the low-acoustic-velocity film stacked on thehigh-acoustic-velocity supporting substrate and disposed under thepiezoelectric film is in a range of about 0.1λ to about 0.5λ, where λ isa wavelength of an elastic wave determined by an electrode period of theIDT electrode; the piezoelectric film includes at least one of LiTaO₃,LiNbO₃, ZnO, AlN, and PZT; and the low-acoustic-velocity film includesat least one of silicon oxide, glass, silicon oxynitride, tantalumoxide, and any material obtained by adding fluorine, carbon, or boron tosilicon oxide.
 2. The elastic wave device according to claim 1, whereina thickness of the piezoelectric film is about 1.5λ or less.
 3. Theelastic wave device according to claim 1, wherein the elastic wavedevice further includes at least one of an adhesion layer, an underlyingfilm, and a medium layer.
 4. The elastic wave device according to claim3, wherein the medium layer is composed of a metal.
 5. The elastic wavedevice according to claim 1, wherein the thickness of thelow-acoustic-velocity film stacked on the high-acoustic-velocitysupporting substrate and disposed under the piezoelectric film is in arange of about 0.2λ to about 0.5λ.
 6. An elastic wave device including apiezoelectric film, the elastic wave device comprising: a supportingsubstrate; a high-acoustic-velocity film disposed on the supportingsubstrate, in which an acoustic velocity of a bulk wave propagatingtherein is higher than an acoustic velocity of an elastic wavepropagating in the piezoelectric film; a low-acoustic-velocity filmstacked on the high-acoustic-velocity film; the piezoelectric filmstacked on the low-acoustic-velocity film; and an IDT electrode disposedon a surface of the piezoelectric film; wherein a thickness of thelow-acoustic-velocity film stacked on the high-acoustic-velocity filmand disposed under the piezoelectric film is in a range of about 0.1λ toabout 0.5λ, where λ is a wavelength of an elastic wave determined by anelectrode period of the IDT electrode; the piezoelectric film includesat least one of LiTaO₃, LiNbO₃, ZnO, AlN, and PZT; and thelow-acoustic-velocity film includes at least one of silicon oxide,glass, silicon oxynitride, tantalum oxide, and any material obtained byadding fluorine, carbon, or boron to silicon oxide.
 7. The elastic wavedevice according to claim 6, wherein a thickness of the piezoelectricfilm is about 1.5λ or less.
 8. The elastic wave device according toclaim 6, wherein the elastic wave device further includes at least oneof an adhesion layer, an underlying film, and a medium layer.
 9. Theelastic wave device according to claim 8, wherein the medium layer iscomposed of a metal.
 10. The elastic wave device according to claim 6,wherein the thickness of the low-acoustic-velocity film stacked on thehigh-acoustic-velocity film and disposed under the piezoelectric film isin a range of about 0.2λ to about 0.5λ.
 11. The elastic wave deviceaccording to claim 1, wherein a material of the high-acoustic-velocitysupporting substrate includes at least one of silicon, sapphire,alumina, magnesia, silicon nitride, aluminum nitride, silicon carbide,zirconia, cordierite, mullite, steatite, forsterite, dielectrics, andgallium nitride.
 12. An elastic wave device comprising: ahigh-acoustic-velocity supporting substrate; a piezoelectric filmindirectly stacked on the high-acoustic-velocity supporting substrate;an IDT electrode disposed on the piezoelectric film; and alow-acoustic-velocity film stacked between the piezoelectric film andthe high-acoustic-velocity supporting substrate; wherein a material ofthe piezoelectric film includes at least one of LiTaO₃, LiNbO₃, ZnO,AlN, and PZT; a material of the low-acoustic-velocity film includes atleast one of silicon oxide, glass, silicon oxynitride, and tantalumoxide; a material of the high-acoustic-velocity supporting substrateincludes at least one of silicon, sapphire, alumina, magnesia, siliconnitride, aluminum nitride, silicon carbide, zirconia, cordierite,mullite, steatite, forsterite, dielectrics, and gallium nitride; and athickness of the low-acoustic-velocity film is in a range of about 0.1λto about 0.5λ, where λ is a wavelength of an elastic wave determined byan electrode period of the IDT electrode.
 13. The elastic wave deviceaccording to claim 12, wherein the low-acoustic-velocity film is stackedon the high-acoustic-velocity supporting substrate.
 14. The elastic wavedevice according to claim 12, wherein a thickness of the piezoelectricfilm is about 1.5λ or less.
 15. The elastic wave device according toclaim 12, wherein the elastic wave device further includes at least oneof an adhesion layer, an underlying film, and a medium layer.
 16. Theelastic wave device according to claim 15, wherein the medium layer iscomposed of a metal.
 17. The elastic wave device according to claim 12,wherein the thickness of the low-acoustic-velocity film is in a range ofabout 0.2λ to about 0.5λ.
 18. An elastic wave device comprising: asupporting substrate; a high-acoustic-velocity film directly orindirectly stacked on the supporting substrate; a piezoelectric filmindirectly stacked on the high-acoustic-velocity film; an IDT electrodedisposed on the piezoelectric film; and a low-acoustic-velocity filmstacked between the piezoelectric film and the high-acoustic-velocityfilm; wherein a material of the piezoelectric film includes at least oneof LiTaO₃, LiNbO₃, ZnO, AlN, and PZT; a material of thelow-acoustic-velocity film includes at least one of silicon oxide,glass, silicon oxynitride, and tantalum oxide; a material of thehigh-acoustic-velocity film includes at least one of silicon, sapphire,or alumina, magnesia, silicon nitride, aluminum nitride, siliconcarbide, zirconia, cordierite, mullite, steatite, forsterite,dielectrics, and gallium nitride; and a thickness of thelow-acoustic-velocity film is in a range of about 0.1λ to about 0.5λ,where λ is a wavelength of an elastic wave determined by an electrodeperiod of the IDT electrode.
 19. The elastic wave device according toclaim 18, wherein the low-acoustic-velocity film is stacked on thehigh-acoustic-velocity film.
 20. The elastic wave device according toclaim 18, wherein a thickness of the piezoelectric film is about 1.5λ orless.
 21. The elastic wave device according to claim 18, wherein theelastic wave device further includes at least one of an adhesion layer,an underlying film, and a medium layer.
 22. The elastic wave deviceaccording to claim 21, wherein the medium layer is composed of a metal.23. The elastic wave device according to claim 18, wherein the thicknessof the low-acoustic-velocity film is in a range of about 0.2λ to about0.5λ.